1. Field of the Invention
The present invention relates to substrates for use in the preparation of printed circuits and the like. In particular, the present invention is directed to a method for improving the adhesion between a conductive laminate and a substrate material. In a preferred embodiment of the present invention, the conductive laminate is a copper film and the substrate material comprises polyphenylene ether resin.
2. Brief Description of the Related Art
Printed circuits are typically prepared by adhering conductive traces which form an electrical interconnect pattern onto a suitable substrate. Commonly-employed substrate materials have included epoxy-glass laminate, paper-phenolic laminate, polyimide-glass laminate, flexible polyimide films, and injection molded plastics such as liquid crystal polymers and polyphenylene sulfide resin. Copper foil has most successfully been used to form conductive traces on these substrates, in particular because of its high electrical conductivity, ready availability, and relatively low cost.
In general, metal-substrate laminates, which can be formed by forming metal by plating or by adhering a metal foil onto substrate, have a tendency to delaminate during and after the formation of a plated metal layer onto the substrate. The peel strength of many such laminates currently in use is generally felt to be insufficient for many end uses because any delamination can cause the failure of the laminate to operate in its intended use. However, even the peel strength currently achievable in many films can be still further decreased by exposure of the film to processing chemicals (etchants, cleaners, coatings, etc.) and environmental stress (such as humidity) and can be reduced to much less than 3 pounds per inch and in certain instance, can be much less than 1 pound per inch. Delamination of the metal layer can result in the failure of the material to be reflective, insulating, an adequate packaging material, or to function in a useful circuit assembled on a printed wiring board made from the laminate.
A variety of other influences can promote the delamination of metal poorly bonded to film substrate. First, the strength of the laminate bond is an important characteristic. Higher strength bonds reduce delamination tendency. Further, the mechanical stresses (soldering, film flex during processing, etc.) involved in first forming the metal on the flexible film and in subsequent processing steps can cause the film to distort or flex and can cause the poorly bonded metal to leave the film.
Additionally, a number of polymer surfaces are known to be less likely to maintain integral laminate structure. Fluorocarbon resins, polyethylene, polypropylene, and polyvinylidiene chloride or polyvinylidiene-fluoride films tend to be difficult surfaces for metal bonding.
Flexible printed circuit boards are currently one preferred circuit manufacturing format used in a variety of electronic devices. These boards are fabricated from flexible plastic substrates having a thin metal laminate layer, generally copper, and can have conductive metal on one or both surfaces with through-hole interconnections. During circuit fabrication, the metal is selectively removed by chemical etching or is pattern plated to leave a pattern of the desired interconnecting circuitry between various components in an electronic circuit. With improvements in etching technology, intercircuit line spacings approaching two-thousandths of an inch can be achieved. Narrow line spacing is one of the current technical innovations that permit continued miniaturization of complex circuitry. However, a narrow line width can promote delamination.
As a result of the problems in laminate preparations and the rigors of the laminate use, an increase in the bond strength of the metal layer to the film polymer substrate is a highly desirable end in the production of inexpensive delamination resistant metal-film laminates.
Kennedy, U.S. Pat. No. 3,700,538 discloses an adhesive used to bond copper foil to resin impregnated fiber glass cloth using a polyimide resin adhesive. The use of an adhesion promoter to bond metal to an insulating base material is also known. For example, Soukup U.S. Pat. No. 3,477,900 and Goepfert et al., U.S. Pat. No. 3,149,021 disclose that when the insulating base material comprises methylmethacrylate resin, an unsaturated polyester may be added to the resin as an adhesion promoter to bind a copper foil. However, these patents disclose that an increase in the proportion of polyester is generally accompanied by a decrease in an adhesion of the copper foil to the resinous base. Barrell et al., U.S. Pat. No. 4,420,509 and Cordts et al., U.S. Pat. No. 4,093,768 disclose procedures for preparing polyester resin copper clad laminates. These processes require several steps or expensive continuously operating equipment.
Van Essen, U.S. Pat. No. 4,081,578; Shanoski et al., U.S. Pat. No. 4,189,517 and Cobbledick et al., U.S. Pat. No. 4,414,173 are directed to in-mold coating processes which are substantially different from the present process in that a preform substrate is either made or placed in a mold and cured. The mold is opened and a small amount of resin is placed on the molded substrate-sufficient to form a coating up to about 20 mils. in thickness. The mold is then closed over the polymerizing resin to apply pressure.
Japanese Patent No. 57083-427 discloses a process where an insulation material is mounted on an inner surface of an injection mold and a metal foil is overlaid on the insulated surface and fixed. A thermoplastic resin is melt-injected into the mold to provide a resin product laminated firmly with the metal foil.
Bristowe et al, U.S. Pat. No. 4,916,016 also teaches injection molded metal-thermoset laminates.
Kawakami et al, U.S. Pat. No. 4,913,938 coating a resin substrate with a copper solution and heating in a non-oxidizing atmosphere to increase-copper laminate adhesion.
Pinch et al, U.S. Pat. No. 4,772,488 teaches the use of a carbon dioxide plasma to treat and clean dielectric layers.
Haque et al, U.S. Pat. No. 4,524,089 uses a three step plasma treatment of copper foils.
Shanefield et al., U.S. Pat. Nos. 4,444,848 and 4,582,564 teach a sputter etching of a rubber modified epoxy surface or coating.
Holmes et al, U.S. Pat. No. 4,153,518, teaches treating a refractory metal oxide layer to improve adhesion of oxide forming metals.
Toth et al, U.S. Pat. No. 4,568,413 teaches forming a releasable metallic layer on a polymeric carrier, adhering the releasable-metal onto a substrate and peeling the carrier.
Sato, U.S. Pat. No. 4,193,849 teaches conventional pre-treatments of plastic prior to electrochemical deposition of metal surfaces.
Ho et al., U.S. Pat. No. 4,720,401 teaches heating a film substrate to a temperature between 0.6 and 0.8 of the curing temperature (Tc) of the substrate material, commonly an elevated temperature exceeding 200.degree. C. (often 240-280.degree. C.) and evaporating or sputtering metal ions such that a metal ion can interact with the heated substrate layer and penetrate into the interior of the heated substrate. The processes in Ho et al are done in an inert atmosphere and produce no metal oxide.
Fronz et al, Plasma Pretreatment of Polyimide Films, a paper presented at the Apr. 24-28, 1989 meeting of the Soc. of Vacuum. Coaters, teach many of the drawbacks of copper-polyimide laminates. Fronz et al teaches that surface cleaning of the polyimide film will increase peel strength. Fronz et al does not discuss the importance of metal-oxide adhesion structures nor uses metallic methods in the film treatment.
Capote et al., U.S. Pat. No. 5,376,403, teaches compositions comprising three or more of the following: a relatively high melting metal powder; a lower melting point metal powder; an active cross-linking agent which also serves as a fluxing agent; a resin; and a reactive monomer.
While many of the aforementioned compositions provide bulk electrical conductivities approaching that of solid copper with resistance to degradation at high temperatures and relative humidities, some of these compositions also do not exhibit entirely adequate adhesion properties when applied to many of the traditional substrate materials. Moreover, many of the processes known in the art require complicated or expensive steps and or equipment.
Accordingly, it is an object of the present invention to provide a method for enhancing the adhesion between a conductive laminate and a substrate material. In a preferred embodiment of the present invention, the conductive laminate is a copper film and the substrate material comprises polyphenylene ether resin.